(Prior Art)
Extremely low carbon steel produce by reducing the carbon content of steel refined in the steel-making process to a few 10 ppm in a vacuum degassing apparatus such as RH, etc., and further by adding carbide-nitride-forming elements such as Ti, Nb, etc. thereto, thereby fixing the remaining carbon, that is, the so called IF steel (interstitial free steel) has a very excellent formability such as deep drawing, etc. and is now much used in automobile application, etc. IF-based, high strength sheet steel, intensified by adding solid solution-strengthening elements such as P, Mn, etc. to the IF steel has also now established a dominant position as the main steel species of high strength sheet steels. However, the IF steel has several process and product drawbacks.
First process drawback is a surface defect. IF steel is vacuum degassed in the steel-making process, and consequently the oxygen content is increased owing to the CO equilibrium, and thus deoxidation is carried out. It is hard to completely remove the deoxidation products, which are liable to remain as inclusions.
Furthermore, solidification in the continuous casting process substantially eliminates carbon, and thus there are no coexisting solid-liquid zones substantially at all. Unstability such as a temperature fluctuation, etc. is directly connected to unstability of steel slab quality, resulting in deterioration of surface properties. Furthermore, the IF steel can be deemed to be nearly pure iron in the composition and has a higher Ar.sub.3 transformation point. Thus, the hot rolling finishing temperature must be made higher, resulting in inevitable generation of surface defects. The serious situation can be understood by a forum "Technique to prevent surface defects in the hot rolling and heavy plate rolling" held in No. 126 Autumn Lecture Conference of Japan Iron and Steel Institute, where the IF steel was taken as the main subject (see, for example, Lecture summary, CAMP-ISIJ, vol. 6, pp 1328-1331 and pp 1332-1335). In spite of the forum, measures to be taken were of on-the-spot type and allopathic and no fundamental solution was made at all yet.
Other drawbacks include several types, depending on differences in species of sheet steels and surface treatment, such as cold rolled steel sheets, electroplated steel sheets, and hot zinc-dipped steel sheets. The IF steel is generally sensitive to the nature of surface treatment. Particularly the iron-zinc alloy coated steel (galvannealed steel) is strongly influenced by steel components. Thus, in case of cold rolled steel sheets and galvannealed steel sheets, it is an ordinary expedient to classify the IF steel, depending on their purposes. For example, Nb-contained IF steel having a better hot dip galvanizing property is preferably used for the galvannealed products, whereas Ti-containing IF steel having a high Lankford value (hereinafter referred to as "a r value") as an indicator of material quality, particularly deep drawability is preferably used for the cold rolled steel sheet, as classified depending on the purposes. This is true also of IF-based high strength steel sheets. However, the minute classification of steel species was against the mass production as the basis of iron and steel industries and largely deteriorated the economy.
Still furthermore, satisfactory metallographic structure cannot be obtained at the heat-influenced parts of welded joints, because the IF steel is extremely low carbon steel. Thus, the IF steel has such a drawback that strength or fatigue characteristics is deteriorated at sites subject to heat influence such as sites subject to spot welding. To overcome the drawback, some measures have been taken, for example, to modify the component system (JP-B-4-2661). However, restriction to the component system to this effect has given rise to another disadvantages such as limited applications or a failure to completely utilize its component system from the viewpoint of workability, etc. and also to an economic loss due to addition of alloying elements.
Many patent applications have been so far filed as to steel sheets comprising a plurality of layers, as in the present invention. For example, JP-A-4-191330 and JP-A-4-191331 disclose that a high strength such as a high dent resistance, etc. can be obtained by using at least one layer made from alloy steel containing a large amount of C, Mn, Si, P, etc.
However, these inventions attempt to obtain novel functions in the strength, formability, etc. by substantially changing mechanical properties of the surface layers and the inner layer. Thus, generally the surface layers have a larger thickness or the surface layers are set to have a higher strength JP-B-6-47706 discloses a technique of surface carburizing of the IF steel, where the product carburized layer has a larger thickness and the technique is not directed to overcoming if process defects as in the present invention.
As to a process for producing a sheet steel comprising a plurality of layers as in the present invention, JP-A-63-108947, for example, discloses a process utilizing a static magnetic field as a means for separating different species of molten steel metals poured into a mold, where the static magnetic field is so formed that a line of magnetic force can be extended at a uniform density over the entire width of molten slab in the direction perpendicular to the casting direction, and different species of molten metals are supplied to the molten slab separated into an upper molten steel pool and a lower molten steel pool by the generated static magnetic field zone as a boundary. As a result of suppressing intermixing of the upper and lower molten steel pools by the static magnetic field, the metal of the upper molten steel pool and the metal of the lower molten steel pool can be separated from each other and solidified individually as a surface layer and an inner layer, respectively, to form a slab having a plurality of layers.
(Problems to be solved by the Invention)
Object of the present invention is to completely overcome various defects of IF-based sheet steels as mentioned above, that is,
1. Surface defects due to the fact that IF sheet steels can be deemed to be nearly as pure iron containing substantially no carbon, PA0 2. Minute classification of steel species into cold rolled steel sheets, electroplated steel sheets and hot dip galvanized steel sheets due to use of or differences in the surface treatment, and PA0 3. Shortage of product strength such a deterioration of fatigue characteristics at welded parts, thereby converting the IF-based sheet steels into a basic material affirmed from the viewpoints of properties and economy. PA0 (1) A continuously cast slab of extremely low carbon steel with less surface defects in a hot rolling, cold rolling, annealing or surface treatment step, which comprises PA0 (2) A continuously cast slab of extremely low carbon steel with less surface defects in a hot rolling, cold rolling, annealing or surface treatment step, as described in item (1), where the surface layer and the inner layer each further contain 0.0001-0.0015% by mass of B. PA0 (3) A continuously cast slab of extremely low carbon steel, as described in item (1), wherein PA0 (4) A continuously cast slab of extremely low carbon steel, as described in item (3), wherein the surface layer and the inner layer each further contain 0.0001-0.0010% by mass of B. PA0 (5) A continuously cast slab of extremely low carbon steel as described in any one of items (1)-(4), wherein total thickness of the surface layer on both sides of the slab is 5-15% of that of the inner layer. PA0 (6) A continuously cast slab of extremely low carbon steel as described in item (5), wherein total thickness of the surface layer on both sides of the slab is 5.0-9.0% of that of the inner layer. PA0 (7) An extremely low carbon sheet steel with less surface defects in steel sheet-producing step, which comprises a sheet steel containing not more than 1.5% by mass of Si, not more than 2.0% by mass of Mn, not more than 0.15% by mass of P, 0.01-0.15% by mass of Al and not more than 0.0050% by mass of N, and PA0 (8) An extremely low carbon sheet steel as describes in item (7), wherein the surface layer and the inner layer each contain 0.0001-0.0015% by mass of B. PA0 (9) An extremely low carbon sheet steel, as described in item (7), wherein PA0 (10) An extremely low carbon sheet steel as described in item (9), wherein PA0 (11) An extremely low carbon sheet steel, as described in item (7), wherein PA0 (12) An extremely low carbon sheet steel, as described in item (11), wherein the surface layer and the inner layer each further contain 0.0001-0.0010% by mass of B. PA0 (13) An extremely low carbon sheet steel, as described in any one of items (7)-(12), wherein total thickness of the surface layer on both sides of the sheet steel is not more than 8% of that of the inner layer. PA0 (14) An extremely low carbon sheet steel, as described in item (13), wherein total thickness of the surface layer on both sides of the sheet steel is 2-8% of that of the inner layer. PA0 (15) A process for producing a continuously cast slab of extremely low carbon steel with less surface defects in a hot rolling, cold rolling, annealing or surface treatment step by continuous casting comprising applying a direct current magnetic field, which crosses the thickness of slab at a lower level position in the casting direction than the meniscus of molten steel poured into a continuously casting mold, to the molten steel, thereby forming a direct current magnetic field zone, and conducting casting while separating the molten steel into an upper molten steel pool and a lower molten steel pool by the direct current magnetic field zone, thereby forming a slab comprising a plurality of layers, where a surface layer and an inner layer have different steel compositions, characterized by pouring molten steel containing not more than 0.0050% by mass of C, not more than 1.5% by mass of Si, not more than 2.0% by mass of Mn, not more than 0.15% by mass of P, 0.01-0.15% by mass of Al and not more than 0.0050% by mass of N, the balance being Fe and inevitable impurities into the continuously casting mold, PA0 (16) A process for producing a continuously cast slab of extremely low carbon steel with less surface defects in a hot rolling, cold rolling, annealing or surface treatment step, as described in item (15), where the surface layer and the inner layer each further contain 0.0001-0.0015% by mass of B. PA0 (17) A process for producing a continuously cast slab of extremely low carbon steel, as described in item (15) or (16), wherein the molten steel poured into the continuously cast mold is made to contain not more than 0.0025% by mass of carbon, and the inner layer is made to contain 0.025-0.040% by mass of Ti, and further a heating temperature for the hot rolling is made to be not more than 1,100.degree. C. PA0 (18) A process for producing a continuously cast slab of extremely low carbon steel, as described in item (17), wherein the inner layer further contains 0.01-0.02% by mass of Nb. PA0 (19) A process for producing a continuously cast slab of extremely low carbon steel, as described tin any one of items (15)-(17), wherein the powder contain 0.5-10% by mass of C. PA0 (20) A process for producing a continuously cast slab of extremely low carbon steel with less surface defects in a hot rolling, cold rolling, annealing or surface treatment step, as described in item (15), by continuous casting comprising applying a direct current magnetic field, which crosses the thickness of slab at a lower level position in the casting direction than the meniscus of molten steel poured into a continuously casting mold, to the molten steel, thereby forming a direct current magnetic field zone, and conducting casting while separating the molten steel into an upper molten steel pool and a lower molten steel pool by the direct current magnetic field zone thereby forming a slab comprising a plurality of layers, where a surface layer and an inner layer have different steel compositions, characterized by PA0 (21) A process for producing a continuously cast slab of extremely low carbon steel as described in item (20), where the surface layer and the inner layer each further contain 0.0001-0.0010% by mass of B. PA0 (22) A process for producing a continuously cast slab of extremely low carbon steel, as described in item (20) or (21), wherein the powder contains 0.5-5% by mass of C. PA0 (23) A process for producing an extremely low carbon sheet steel with less surface defects in a steel sheet-making step by continuous casting comprising applying a direct current magnetic field, which crosses the thickness of slab at a lower level position in the casting direction than the meniscus of molten steel poured into a continuously casting mold, to the molten steel, thereby forming a direct current magnetic field zone, and conducting casting while separating the molten steel into an upper molten steel pool and a lower molten steel pool by the direct current magnetic field zone, thereby forming a slab comprising a plurality of layers, where a surface layer and an inner layer have different steel compositions, characterized by PA0 (22) A process for producing an extremely low carbon sheet steel, as described in item (23), wherein the surface layer and the inner layer each further contain 0.0001-0.0015% by mass of B. PA0 (25) A process for producing an extremely low carbon sheet steel, as described in item (23), wherein molten steel containing not more than 0.0050% by mass of C, 0.05-0.40% by mass of Mn, 0.01-0.10% by mass of Al and not more than 0.0050% by mass of N, the balance being Fe and inevitable impurity elements, is produced and then subjected to continuous casting, thereby obtaining a slab while providing an electromagnetic brake at a mold, PA0 (26) A process for producing an extremely low carbon sheet steel, as described in item (25), wherein molten steel containing not more than 0.0050% by mass of C, 0.05-0.40% by mass of Mn, 0.01-0.10% by mass of Al, not more than 0.0050% by mass of N and 0.0001-0.0010% by mass of B, the balance being Fe and inevitable impurity elements, is produced and then subjected to continuous casting, thereby obtaining a slab, while providing an electromagnetic brake at a mold, PA0 (27) A process for producing an extremely low carbon sheet steel with less surface defects in a steel sheet-producing step, as described in item (23), by continuous casting comprising applying a direct current magnetic field, which crosses the thickness of slab at a lower level position in the casting direction than the meniscus of molten steel poured into a continuously casting mold, to the molten steel, thereby forming a direct current magnetic field zone, and conducting casting while separating the molten steel into an upper molten steel pool and a lower molten steel pool by the direct current magnetic field zone, thereby forming a slab comprising a plurality of layers, where a surface layer and an inner layer have different steel compositions, characterized by PA0 (28) A process for producing an extremely low carbon sheet steel, as described in item (27), wherein the surface layer and the inner layer each further contain 0.0001-0.0010% by mass of B. PA0 (29) A process for producing an extremely low carbon sheet steel, as described in any one of items (23)-(28), wherein the ordinary hot rolling--pickling--cold rolling is carried out and the recrystallization annealing and galvanizing are carried out in a continuous hot dip galvanizing line. PA0 (30) A process for producing an extremely low carbon sheet steel, as described in any one of items (23)-(29), wherein the treatment in the continuous hot dip galvanizing line is carried out by galvanizing and then an ordinary alloying treatment to zinc phase. PA0 (31) A process for producing an extremely low carbon sheet steel, as described in any one of items (23)-(30), wherein the hot rolling is carried out on the continuously cast slab in the ordinary manner comprising subjecting the slab to direct hot rolling at 1,050.degree.-1,200.degree. C. or heating--rough rolling--finish rolling, where the finish rolling end temperature is above the Ar.sub.3 tranformation point or may be below the Ar.sub.3 transformation point within a range of evading a ridging-like skin toughening, followed by cooling, coiling at about 550.degree.-690.degree. C., further cooling, pickling if required and an appropriate finishing treatment, thereby obtaining a hot rolled steel sheet or hot rolled coil. PA0 (32) A process for producing an extremely low carbon sheet steel, as described in item (31), wherein the finish rolling end temperature is [Ar.sub.3 transformation point--20] -950.degree. C. PA0 (33) A process for producing an extremely low carbon sheet steel, as described in any one of items (23)-(32), wherein in the hot rolling--pickling--cold rolling--recrystallization annealing, cold rolling is carried out in the ordinary manner comprising PA0 (32) A process for producing an extremely low carbon sheet steel, as described in any one of items (23)-(33), wherein in the hot rolling--pickling--cold rolling--surface treatment, the surface treatment is carried out in the ordinary manner comprising passing the cold rolled coil through an electrogalvanizing line or an electrogalvanizing-alloying line, thereby obtaining a pure zinc coated steel sheet or zinc alloy coated steel sheet. PA0 (35) A process for producing an extremely low carbon sheet steel, as described in item (34), wherein the electrogalvanizing is an ordinarly pure zinc plating. PA0 (36) A process for producing an extremely low carbon sheet steel, as described in item (34), wherein the electrogalvanizing-alloying is plating of Zn--Ni alloy containing zinc as the major component. PA0 (37) A process for producing an extremely low carbon sheet steel, as described in any one of items (23)-(33), wherein in the hot rolling--pickling--cold rolling--surface treatment, the surface treatment is carried out in the ordinary manner comprising passing the cold rolled coil through a continuous hot dip galvanizing line under conditions of 700.degree.-900.degree. C. for 10 sec.-10 min., thereby obtaining a hot dip galvanized steel sheet. PA0 (38) A process for producing an extremely low carbon sheet steel, as described in any one of items (23)-(33) and (37), wherein in the hot rolling--pickling--cold rolling--surface treatment, the surface treatment is carried out in the ordinary manner comprising